Step motors are commonly used to adjust a position of a lens or a movable component such as a pickup or a head, drive a paper transfer roller, or the like, in electronic devices such as digital cameras, digital video cameras, disk devices, printers, or copy machines. Step motors, which are synchronous motors rotating in synchronization with a step pulse signal applied from the outside, have excellent controllability in terms of startup, stop, and position determining. In addition, step motors have characteristics that they can be controlled in an open loop and are appropriate to process digital signals.
In a normal state, a rotor of a step motor rotates in synchronization with a step pulse signal. However, if there is an overload or an abrupt change in speed, the synchronization between the step pulse signal and the rotation of the rotor is broken. This is known as step-out (or synchronization secession). Once stepped out, the step motor should undergo special processing so as to be normally driven, and thus, it is preferred to prevent the step motor from stepping-out.
In order to address the problem, in many cases, a motor driving circuit is designed to obtain an excessive output torque having a margin based on an anticipated maximum load, but loss of power is increased. Further, in order to detect a position of the rotor, a sensor is used or a vector is controlled by using a large scale digital calculation circuit such as an A/D converter, but in this case, cost is increased due to an increase in the number of components and a logical area is also increased.